1. Field of the Invention
The present invention relates to pipe manufacturing processes. In particular, the present invention provides a method and apparatus for sizing newly formed steel pipe in a stretch reduction mill manufacturing process.
2. Description of the Related Art
Steel pipe is commonly manufactured by various multi-step techniques, all of which include starting with an input product, which may be a welded tube or bar or rod stock, and sizing that product using one or more sizing techniques to create the final desired pipe size and configuration. One sizing technique is to feed input tubes through a hot stretch reduction mill, which produces tubing that may be at its final desired size, or may become input stock for further sizing processes, depending upon the intended final product and application.
In a hot stretch reducing mill, heated tube stock is fed through one or more matched sets of roll stands. Each stand has three contoured rolls that rotate at a predetermined, stand-specific speed. Changes in roll contour determine the overall diameter reduction, while changes in stand roll speed determine final tube wall thickness. Typically, as a heated input tube progresses through the stands, each stand progressively reduces the tube until it reaches the specified exit diameter and wall thickness at the end of the stands. At each stand, the roll speed (revolutions per minute) is progressively increased, causing the heated tube to elongate and increase its linear speed. Note, however, that roll speed may also be decreased, causing an increase in tube thickness.
The hot stretch reduction mill process is typically a smooth, uninterrupted process that consists of first heating the input tube, feeding the heated tube through the stands that stretch and reduce the tube, and then cutting the output tube to length by a flying cut-off saw.
In current hot stretch reduction mills, the drive system for the rolls in each roll stand is typically a hydraulically-driven drive motor, coupled to a hydraulically-driven speed control motor that uses a swash plate speed control device. This arrangement has a number of drawbacks, including imprecise speed control capability (which is exacerbated by wear in the hydraulic units), increasingly frequent maintenance as the drive system ages, and generally high operation and maintenance costs. Perhaps most importantly, the hydraulic speed control motors' feedback time delay is typically too large to maintain effectively the stand's pre-set roll speed. Consequently, setting the speed control motors to produce pipe of the correct dimensions is a process of trial and error--running pipe through the mill, measuring the final product, and adjusting the motors--that results in wasted pipe that must be re-melted and processed.
The present invention overcomes these drawbacks by replacing the hydraulic speed control motor and swash plate speed control device in each stand's drive system with an electronic stand controller and an electric motor in a feedback loop. Each stand controller couples to a master controller. Speed pickup devices at each stand continuously monitor final stand rotation speed and send that information to the master controller. The master controller compares those measured values with the desired stand speed at each stand, and provides signals to the individual stand controllers that adjust the stand motor speed as required to maintain the desired stand speed. The master controller that controls the entire system interfaces with the operator through a standard desktop computer workstation. The present invention is thus a method and apparatus that precisely and continuously controls the roll speed of a plurality of roll stands in tube reducing mills. The present invention improves product quality by reducing product size variation and reduces operations costs associated with repairing hydraulic fluid leaks and reduces other maintenance costs.